1. Field of the Invention
This invention relates to an optical external modulator to be used for external modulation of the light being propagated through an optical transmission line of an optical telecommunications system.
2. Prior Art
Conventional optical modulators are mostly of the direct modulation type where the light emitted from a light source such as a semiconductor laser device or a light-emitting diode is modulated by directly modulating the electric current being fed to the light source. However, optical external modulators have recently been proposed to indirectly modulate the light being emitted from a source of continuous light and propagated through an optical transmission line (e.g., optical fiber) by externally applying signals to the light.
Japanese Patent Application No. 3-196291 teaches such an optical external modulator. Referring to FIG. 26(a) of the accompanying drawings, it comprises a lower electrode 2, a piezoelectric film 3 and an upper electrode 4 sequentially arranged in the described order on a side of a substrate 1 of quartz glass to produce a thin filmlike piezoelectric device 5 having a multilayer structure as well as lead wires 10a and 10b connected respectively to the lower and upper electrodes 2 and 4 in order to feed the piezoelectric film 3 with modulation signals and drive it to operate. For use, as illustrated in FIG. 26(b), a single mode optical fiber 17 is fitted to the side oil the substrate opposite to the one where the electrodes 2 and 4 are arranged at a position directly below the piezoelectric film 3 and rigidly secured to the substrate 1 by means of a sheath 9 having a specific: acoustic impedance (the density of a medium to be used for the propagation of sound waves multiplied by the velocity at which the sound is propagated through tile medium) close to that of the clad of the optical fiber 17 in order to cover the latter for a desired distance. FIG. 27 shows another conventional optical external modulator. A single mode optical fiber 17 is arranged directly on the upper electrode 4 of the thin filmlike piezoelectric device 5 of tile modulator and rigidly secured to the substrate 1 by means of a sheath 9 having a specific acoustic impedance close to that of the clad of the optical fiber 17, lead wires 10a and 10b being respectively connected to the upper and lower electrodes 2 and 4 in order to feed the piezoelectric film 3 with modulation signals and drive it to operate.
With any of the above described optical external modulators, the piezoelectric film 3 periodically generates an elastic wave as a modulation signal having a predetermined frequency is applied to the appropriate one of the lead wires 10a and 10b from a drive power source and the stress given rise to by the wave is applied in turn to the single mode optical fiber 17 by way of the substrate 1 to produce a specific internal distribution pattern of refractivity variances within the optical fiber 17, which accordingly modifies the state of polarization of the light passing through the optical fiber 17.
The above described optical external modulators are of the so-called optical fiber type where a modulator (comprising a substrate 1 and a thin filmlike piezoelectric device 5) is combined with a single mode optical fiber to form a unit and hence not accompanied by any insertion loss that may become existent if the single mode optical fiber is connected to the modulator at a later stage.
The performance of such an optical external modulator can be determined by means of a gauging system as illustrated in FIG. 28. Light emitted from a light source such as a laser diode (LD) is made to pass through a polarizer designed to optimize the state of polarization of the incoming light and then introduced into an optical external modulator 33. A modulation signal having a predetermined frequency is applied to the optical external modulator 33 from a drive power source 34 in order to modify the state of polarization of the light being propagated through the single mode optical fiber in terms of the polarization. The mode of modulation of the light is then converted to that of intensity by an optical analyzer 35 and the light is thereafter converted into an electric signal by means of an O/E (opto-electric) converter 36. The electric signal is then observed by means of a spectrum analyzer 39 and an oscilloscope 37 to determine the performance of the optical modulator.
Problems to be Solved by the Invention
External optical modulators of the above described type, however, have major drawbacks. Firstly, the incident light cannot be modulated for the polarization at all if the incident light has the axis of polarization that is parallel or vertical to the direction along which stress is applied to the optical fiber, although optical external modulators are perpetually subjected to changes in the state of polarization of the incident light in actual optical telecommunications systems. Therefore, a conventional optical external modulator 33 has to be used in combination with a polarizer 32 arranged immediately upstream to the modulator in order to regulate the state of polarization of the incident light in such a way that the axis of polarization of the incident light may never become horizontal nor vertical to the direction along which stress is applied to the optical fiber. However, such an arrangement for constantly regulating the state of polarization of the incident light for an optical external modulator inevitably involves a large system whose cost will be inhibitive. Thus, the polarization dependency of existing optical external modulators provides a grave problem to be solved if they are used for practical applications.
It is, therefore, an object of the present invention to solve the above identified problems and other problems by providing an optical external modulator that can effectively modulate the incident light in terms of the polarization without requiring a process of regulating the state of polarization of the light incident to the modulator.